Secondary batteries used in vehicle-mounting uses, etc. are expected to be used with high currents. If a secondary battery such as a lithium-ion battery is subjected to continuous charge/discharge with a high current for a long period of time, an internal resistance value is increased, performance is deteriorated, and the performance originally possessed by the secondary battery cannot be fully exerted. Therefore, in order to prevent such performance deterioration, PTL 1 discloses a battery controlling method that accumulates evaluation values, which indicate the deterioration degree of a battery caused by continuance of discharge of the battery, and, if the accumulated value thereof exceeds a predetermined permissible value, restricts discharge of the battery.
In the battery controlling method disclosed in this PTL 1, whether the electric power is to be restricted or not is determined by estimating changes in unevenness of the ion concentration in electrolyte of the secondary battery based on the history of charge/discharge currents. In order to estimate the unevenness of the ion concentration, parameters such as currents and the temperature of the battery have to be used. It is general to acquire the temperature of the battery by attaching a thermocouple or a thermistor to a surface of the battery, and it is extremely difficult to precisely measure the temperature in the vicinity of an electrolysis solution or an electrode directly related to unevenness of ions. In a steady state with no temperature changes, differences among measurement locations do not cause big problems since the surface temperature of the battery and the temperature in the battery are equal. However, if the temperature is changed due to heat generation of the battery caused by charge/discharge, a cooling medium, or a cooling wind, a temperature difference is caused between the inside and outside the battery, and it becomes difficult to precisely estimate the temperature in the battery. As a result, the charge/discharge of the battery may be excessively restricted due to the difference between a measured value of the battery temperature and the actual temperature in the battery, or charge/discharge may not be appropriately restricted, which may lead to resistance increase and unsatisfaction of an expected life.